A polyester (particularly, polyethylene terephthalate (hereinafter, sometimes, referred to as PET), poly(ethylene-2,6-naphthalenedicarboxilate), etc.) resin is superior in mechanical properties, thermal properties, chemical resistance, electrical properties and molding properties, and is used in various applications. A polyester film prepared by forming the polyester into a film, particularly, biaxially oriented polyester film, is used as various industrial materials such as copper clad laminate plates, solar-cell back sheets, pressure-sensitive adhesive tapes, flexible printed boards, membrane switches, sheet heating elements, electrical insulation materials of a flat cable or the like, magnetic recording materials, capacitor materials, packaging materials, automobile materials, building materials, photographic applications, graphic applications and heat-sensitive transfer applications because of its mechanical properties and electrical properties.
Among these applications, in electrical insulation materials (e.g., solar-cell back sheet, etc.), automobile materials and building materials, which are used particularly outdoors, are often used under severe environments in terms of temperature and humidity for a long period. Since in a versatile polyester, its molecular weight is decreased by hydrolysis and embrittlement proceeds to cause mechanical properties to deteriorate, improvement of the polyester, that is, improvement of the moisture-heat resistance, is required. Moreover, in order to prevent the fire spread in the occurrence of a fire disaster, the flame retardancy is required.
Accordingly, various investigations have been made to suppress the hydrolysis of polyester. For example, a technique, in which an antihydrolysis segment such as epoxy-based compounds (Japanese Unexamined Patent Publication No. 9-227767 and Japanese Unexamined Patent Publication No. 2007-302878) or polycarbodiimides (Published Japanese Translation No. 11-506487 of the PCT Application, Japanese Unexamined Patent Publication No. 9-7423 and Japanese Unexamined Patent Publication No. 2003-41030) is added to improve the moisture-heat resistance of polyester, is investigated. Further, with respect to a biaxially oriented polyester film, a technique of improving the moisture-heat resistance by increasing the inherent viscosity (IV) of the film and controlling the planar orientation is investigated (Japanese Unexamined Patent Publication No. 2007-70430).
On the other hand, it is desired to impart properties other than the moisture-heat resistance (e.g., a sliding property, ultraviolet light resistance, a reflecting property, etc.) to increase performance for these applications. Thus, investigations concerning mixing other component (e.g., inorganic particle, etc.) to enhance the functions are made (e.g., Japanese Unexamined Patent Publication No. 2003-155403, Japanese Unexamined Patent Publication No. 2-163155, Japanese Unexamined Patent Publication No. 2-191638 and Japanese Unexamined Patent Publication No. 2006-270025).
However, when other components (e.g., ultraviolet absorber, inorganic particles, etc.) are mixed to enhance the function of a polyester film, particularly a polyester film containing an ethylene terephthalate unit as a main constituent, degradation of a resin proceeds due to hydrolysis in kneading, and the resulting film exhibits the function of the added component, but there is a problem that moisture-heat resistance is deteriorated.
Further, a problem that addition of the antihydrolysis segment causes the reduction of the flame retardancy arises.
Accordingly, it could be helpful to provide a polyester film which is superior in moisture-heat resistance, flame retardancy and ultraviolet light resistance.
We thus provide a polyester film having a laminate structure comprising a polyester layer (layer P1) containing a crystalline polyester and inorganic particles and a polyester layer (layer P2) containing a crystalline polyester, inorganic particles, and an antihydrolysis segment, wherein the content Wa2 of the inorganic particles in the layer P2 is at least 10 mass % of the layer P2, the difference between the content Wa2 (mass %) of the inorganic particles in the layer P2 and the content Wa1 (mass %) of the inorganic particles in the layer P1, Wa2−Wa1, is 5 to 25 mass %, and the content Wb2 of the antihydrolysis segment in the layer P2 is 0.02 to 1.5 mass % of the layer P2.
It is possible to provide a polyester film which satisfies moisture-heat resistance, flame retardancy, and ultraviolet light resistance over a long period. Moreover, it is possible to provide a solar-cell back sheet which uses the polyester film and, hence, has high durability; and a solar cell including the back sheet.